The present invention relates to a cooled mirror, and also to a method of cooling a mirror. The invention is particularly useful as a laser mirror, such as described in U.S. Pat. No. 3,942,880, and is therefore described below with respect to such application.
Lasers of increasing power outputs are now being developed particularly for use in metal-working and other industrial applications. The cooling of the mirrors in such high-power lasers is one of the problems limiting their size since 1-2% of the laser power is absorbed in each mirror. Thus, a laser of 10 kWatts output absorbs from 100-200 watts, which creates serious problems particularly in the thermal deformation of the mirrors since such deformation must be held to very small tolerances for the proper operation of the laser.
U.S. Pat. No. 3,942,880 describes a cooled laser mirror addressed particularly to this problem. The laser therein described includes a reflector face, a base, and a plurality of spaced slender, supporting elements joining the reflector face to the base such as to provide a high resistance to thermal deformation tending to cause bending of the reflector face. That is to say, the slender supporting elements, which extend perpendicularly to the reflector face, constrain the deformation caused by thermal expansion to the direction of the plane of the reflector face, i.e., normal to the axes of the slender supporting elements, and substantially reduce or eliminate thermal deformation axially of the slender supporting elements tending to bend the reflector face. The patent describes these slender supporting elements as rods or wires. It further describes the provision of heat-conductor bars extending from the reflector face between and spaced from the supporting rods, and means for feeding a cooling fluid, such as water, through the spaces between the supporting rods and the heat-conductor bars.
The amount of heat that can be dissipated by the arrangement of this patent, however, even when a cooling fluid is used, is still very limited from a practical aspect when considering practical rates of flow of the cooling fluid. Moreover, the cooling effected by such fluid is not uniform, thereby further contributing to thermal bending of the mirror, e.g., when used in high power lasers. Furthermore, in many higher-power lasers now being designed, the laser beam is non-uniform in cross-section, e.g., of annular configuration, which results in an uneven distribution of the power to be absorbed by the mirror, thereby further increasing the bending deformation caused in the mirror by the uneven thermal expansion.
An object of the present invention is to provide a cooled mirror structure similar to that of U.S. Pat. No. 3,942,880, but which has a far greater capability of dissipating the energy absorbed by the mirror, and therefore, of reducing the bending deformation caused by thermal expansion of the mirror, thereby making the cooled mirror particularly suitable for high-power lasers. Another object of the invention is to provide a method of cooling mirrors or other members, particularly useful for cooling laser mirrors.